1. Field of the Invention
This invention relates to construction of an electrically heated vacuum furnace and techniques for accelerating the heating-up of the inside of a heating chamber or hot zone at lower temperatures. More specifically, the invention concerns the creation of a convection circulation chamber external to the hot zone in such a way that both the heating and cooling processes are improved.
2. Description of the Prior Art
Vacuum furnaces have been commonly used in high temperature thermal treatment of materials requiring a high degree of purity. A typical vacuum furnace consists basically of a hermetic furnace housing inside of which is a heating chamber having a plurality of electrical heater elements suitably mounted on its inside walls. During vacuum heating, air in the furnace is evacuated and the load is heated by radiant heat from the heater elements.
Many types of load also require rapid cooling or quenching in order to improve their metallurgical properties. The quenching process, in some cases, can be critical in determining the quality of the final product. A common method for quenching is by subjecting the load in the hot zone to a flow of pressurized cold inert gas introduced into the heating chamber via gas nozzles or orifices through the heating chamber walls. A product brochure entitled "Industrial Furnace Systems" by GM Enterprises illustrates some typical high temperature vacuum furnaces.
A problem with this kind of vacuum furnace is that it takes a relatively long period of time to heat up the hot zone to its high operative temperatures (for example, around 2000.degree. F.). This is because at lower temperatures (below about 1200.degree. F.) radiation is not an efficient method of heat transfer. Convection is, however, by far the more efficient heat transfer mechanism at lower temperatures.
Therefore, in order to accelerate the heating up of the hot zone, some vacuum furnaces are known to have used convection heating followed by evacuation and radiation heating.
U.S. Pat. No. 4,970,372 (Ipsen Industries International) discloses a furnace with a plurality of electrically energized heater elements disposed within a heating chamber. To accelerate the heating up of a load that is disposed within the heating chamber in the lower temperature range, the heater elements have a tubular construction and are provided at their bottom end with an outlet opening. Inert gas conveyed by a fan flows through the heater elements, at which time it is convectively heated, and thereafter passes into the interior of the heating chamber. In the higher temperature range, the heater elements transfer their heat to the load as radiant heat, with the furnace then operating in a vacuum.
This design suffers from several disadvantages: inert gas flow may be impeded by the size of the heater element; the hollow tubular design of the heater elements are more expensive to produce and less durable than the regular solid type.
In another design, disclosed in a product brochure "High Pressure Quench Vacuum Furnaces" by Abar Ipsen, the heating chamber is partitioned by walls to create an annular passage between the heater elements supported from the interior wall of the heating chamber and the principal furnace insulation. A fan mechanism located at one end of the passage forces inert gas circulation between the interior load area and the heating chamber through openings at opposite ends of the passage. During low temperature heating, inert gas is introduced into the heating chamber such that heat transfer from the heater elements to the load occurs mainly by convection. At higher temperature, heat transfer is achieved by radiation with the furnace operating in a vacuum.
This design overcomes some of the drawbacks presented by the previous design with hollow heater tubes but, nevertheless, creates some problems of its own. One of the major problems has to do with the location of cooling gas nozzles. During a rapid cooling cycle (required for many types of load), it is extremely important to maintain uniform cooling of the load in order to achieve uniformity of physical properties and to prevent physical distortion due to unequal rates of contraction of the load. Uniformity of cooling, however, can be achieved only if it is possible to expose the load to the unobstructed flow of cooling gas in all directions. This stringent requirement, therefore, requires maximum flexibility in the location of the cooling gas nozzles with respect to the load under treatment. The presence of the partition walls reduces such flexibility and may adversely affect the ultimate quality of the load under treatment. This design attempts to remedy the drawback by using movable gas nozzles. However, such a design is more costly to implement due to the higher level of engineering complexities in having moving parts which operate in extremely high temperatures.